China is developing rapidly economically; we are playing an increasingly crucial role on the world scene. As a nation, China is proud of its technical and cultural heritages in the past. To further improve our international visibility, we must invest in science and technology, in particular basic science, which provides the fertile soil for applied technologies to prosper not only at present but also in the future.

As mentioned before, the Chinese astronomical community has made much progress in the last decade with rapid advances in observing facilities such as LAMOST. The research environment (in terms of hardware) is also reaching the western standard. Chinese astronomers are engaged in diverse research areas with a potential pool of excellent students and invaluable resources of oversea Chinese astronomers both in observational and theoretical astrophysics. However, a world class, general-purpose optical-infrared telescope is still lacking: our largest optical telescope in Lijiang, Yunnan has a diameter of 2.4m; this is in sharp contrast with many state-of-the-art 8-10m telescopes throughout the world. Even a third-world country such as South Africa has built their own 10m telescopes. This lack of access to modern telescopes severely hampers our ability to maximize the science returns of China's own large astronomy projects, such as LAMOST and FAST etc.
In this context, the Chinese Astrophysics Strategy Committee listed the participation in the TMT as the highest priority item for the future development of Chinese astronomy. China has gained invaluable experience in the large scientific engineering project LAMOST that has an aperture of about 4m. However, to make the next quantum leap in terms of aperture to 30m, China cannot go alone for several reasons. First, the 30m telescope science requires the best observing site in terms of seeing, sky background, photometric nights etc., in the world, so far no such site has been identified within China. Second, much of the science of 30m class telescopes is in infrared (e.g., for studying the high-redshift universe and the detection of extrasolar planets), and China has no access to infrared CCD technology. Third, the TMT project requires a multitude of expertise in science, technology, engineering and management; such teams are not yet mature within in China. Lastly, the cost involved is prohibitive for any single country. All the proposed 30m class telescopes (TMT, GMT, and E-ELT) involve large international collaborations. The Chinese participation in TMT will bring a number of scientific, managerial, technological and industrial benefits to China:

It will thrust China into the forefront of astronomy in the extremely large telescope era; it will establish a platform for China to collaborate and compete with world astronomers.

It will also enable other large Chinese science projects such as LAMOST and FAST to realise their fullest science capabilities and potentials.

It will enable China to learn key technologies in advanced optics, mechanics, electronics and automation, including technologies in IR CCDs that are inaccessible currently.

State-of-the-art facilities TMT will prove to be a magnet to attract high-level Chinese astronomers to collaborate with Chinese astronomers (and possibly return to China). This will further expand the talent pool that will be crucial for the future success of Chinese astronomy.

TMT will also provide an important platform for Chinese astronomers to collaborate with world astronomers to access other state-of-the-art facilities in other wavelengths, such as SKA (in the radio), ALMA (in the sub-mm), and JWST (optical and infrared, but in space). Such multi-wavelength observational approaches are becoming increasingly important in astrophysics.

It is important to stress that China will not only be able to build specific instruments as in-kind contributions, but also send delegates to learn key technologies in virtually all areas.

The final TMT design/construction readiness review will be in June 2010. It will have its first light with full primary mirror in October 2017, and the first science will be performed in June 2018. The full TMT partnership will be formed in about two years (from December 2009). The later we join, the less China will be able to influence and define our own technical and scientific contributions to the TMT project. It is crucial that our technological capabilities are matched by our success in scientific programs. This can only be ensued if a strong and internationally competitive science team is built up. The rapid emergence of Chinese astronomy on the world scene in the last decade lends confidence that, with appropriate planning and a united effort from the Chinese astronomy community, this can be achieved in the next decade.

3Overview of TMT Observatory and Instruments

3.1TMT Observatory

The core of the TMT Observatory (Figure 1) will be a wide-field, alt-az Ritchey-Chretien telescope with a 492 segment, 30m-diameter primary mirror, a fully active convex secondary mirror and an articulated flat tertiary mirror. The optical beam of this telescope will feed a constellation of adaptive optics (AO) systems and science instruments mounted on large Nasmyth platforms surrounding the telescope azimuth structure. These platforms will be large enough to support at least eight different AO/instrument combinations covering a broad range of spatial and spectral resolution.

Figure 1 The telescope design (left), and the entire observatory system (right). Taken from the TMT web site (tmt.org).The TMT aperture size (30 meters) occupies an attractive and achievable scientific “sweet spot” at near-infrared wavelengths. It is important to stress that the cost for TMT has been reduced from the usual volumetric scaling with aperture of D2.7 to D1.15 due to the past experience accumulated from the successful construction of KECK.
TMT will be the first ground-based astronomy telescope designed with adaptive optics (AO) as an integral system element1. AO is a term that covers systems designed to sense atmospheric turbulence in real-time, correct the optical beam of the telescope to remove its affect, and enable true diffraction-limited imaging on the ground. TMT adaptive optics design builds on the technological and operational heritage of (among others) the Gemini, Keck, and Very Large Telescope observatories. This is an area of rapid advancement and the TMT project has direct access to world leaders in this area (e.g. the Center for Adaptive Optics at the University of California, Santa Cruz). For point sources, the adaptive optics improves the gain in the observing time from D2 to D4 (due to the decrease in the footprint of the Point Spread Function [PSF]) – implying a reduction in observing time for a factor of ~100 rather than a factor of 10 compared with KECK. This vast improvement in efficiency will enable exciting science to be performed on the nearby and distant Universe inaccessible by any other observatory, on the ground or in space.